Mirror

A
mirror is a reflective surface that is
smooth enough to form an image. The best known example is the plane mirror.
The most common use is in the home for shaving, make-up etc but mirrors are also
used in scientific apparati such as telescopes and lasers, and in industrial machinery.

Effect

In
this diagram plane waves reflect off a parabolic mirror to form waves converging
onto a focal point

In
a plane mirror, a parallel beam of light changes its direction as a
whole, whilst still remaining parallel; the images formed by a plane mirror are
virtual images, of the same size as the original object (see mirror image).
There are also parabolicconcave mirrors,
where a parallel beam of light becomes a convergent beam, whose rays
intersect in the focus of the mirror. Finally, there
are convex mirrors, where a parallel
beam becomes divergent, with the apparent intersection occurring behind the mirror.
Note that spherical concave and convex mirrors do not have a single focal point,
as often described in high school physics text books (see spherical
aberration in Lens (optics) and Aberration in
optical systems).

A
beam of light reflects off a mirror at an angle of reflection that is equal to
its angle of incidence. That is, if the beam of light is shining on a mirror's
surface at a 30° angle from vertical, then it reflects from the point of incidence
at a 30° angle from vertical in the opposite direction.

Contrary
to popular belief, mirrors do not actually reverse left-to-right; they actually
reverse front-to-back. Light rays reflecting off a mirror have their front-to-back
direction reversed, while the left-to-right and top-to-bottom components of their
direction remain unchanged. The confusion comes from the perception that if a
person were standing behind the mirror facing us, as our reflection seems to,
then they would be reversed left-to-right relative to ourselves. However, this
left-to-right reversal is not performed by the mirror, but rather by this imaginary
person turning around left-to-right to face us from behind the mirror.

Composition

Most
modern mirrors consist of a thin layer of aluminium deposited on a sheet
of glass. They are
back silvered, where the reflecting surface is viewed through the glass
sheet; this makes the mirror durable, but lowers the image quality of the mirror
due to extraneous reflections from the front surface of the glass. This type of
mirror reflects about 80% of the incident light. The "back side" of the mirror
is often painted black to completely seal the metal from corrosion.

Applications

Reflections
in a spherical convex mirror. The photographer is seen at top right

Viewing
one's own body

A
mirror is used for inspecting parts of one's body which are difficult or impossible
to see directly, such as the face, neck or the whole body. This may be to check
physical appearance
(including clothing, make-up, hair, etc.) or to control applying
make-up, shaving, cutting hair, fixing one's tie, etc.

Instruments

Telescopes and other precision
instruments use front silvered mirrors, where the reflecting surface is
placed on the front surface of the glass, which gives better image quality. Some
of them use silver, but most are aluminum, which is more reflective at short wavelengths
than silver. All of these coatings are easily damaged and require special handling.
They reflect 90% to 95% of the incident light when new. The coatings are typically
applied by vacuum deposition. A protective overcoat is usually applied before
the mirror is removed from the vacuum, because the coating otherwise begins to
corrode as soon as it is exposed to oxygen and humidity in the air. Front silvered
mirrors have to be resurfaced occasionally to keep their quality.

The
reflectivity of the mirror coating can be measured using a reflectometer
and depends on the wavelength of light as well
as the metal. This is exploited in some optical work to make cold
and hot mirrors. A cold mirror is made by using a transparent substrate
and choosing a coating material that is more reflective to visible light and more
transmissive to infrared light. A hot mirror
is the opposite, the coating preferentially reflects infrared. Mirror surfaces
are sometimes given thin film overcoatings both to retard degradation of the surface
and to increase their reflectivity in parts of the spectrum where they will be
used. For instance, aluminum mirrors are commonly coated with magnesium fluoride.
The reflectivity as a function of wavelength depends on both the thickness of
the coating and on how it is applied.

For
scientific optical work, dielectric mirrors
are often used. These are glass (or sometimes other material) substrates on which
one or more layers of dielectric material are deposited,
to form an optical coating. By careful
choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from
the mirror can be specified. The best mirrors of this type can reflect >99.999%
of the light (in a narrow range of wavelengths) which is incident on the mirror.
Such mirrors are often used in lasers.

Safety
and easier viewing

Rear-view
mirrors are applied in and on vehicles (such as cars, or bicycles),
to allow drivers to see other vehicles coming up behind them.

Some
motorcycle helmets have
a built-in so-called MROS (Multiple Reflective Optic System): a set of reflective
surfaces inside the helmet which together function as a rear-view mirror [1] (http://www.reevu.nl).

There
exist rear view sunglasses, of which the left
end of the left glass and the right end of the right glass work as mirrors.

Rounded
mirrors are sometimes placed at corners of places such as parking lots or
stores, allowing people to see around corners to avoid crashing into other vehicles
or shopping carts.

Mirrors
are also sometimes used as part of security systems, so that a single video camera
can show more than one angle at a time.

One-way
mirror

A one-way
mirror, also called two-way mirror (!), reflects about half of the
light and lets the other half pass. It is a sheet of glass coated with a layer
of metal only a few dozen atoms thick, allowing some of the light through the
surface (from both sides). It is used between a dark room and a brightly lit room.
Persons on the brightly lit side see their own reflection -- it looks like a normal
mirror. Persons on the dark side see through it -- it looks like a transparent window.
It may be used to observe criminal suspects or customers (to watch out for theft).
The same type of mirror, when used in an optical instrument,
is called a half-silvered mirror. Its purpose is to split a beam of light
so that half passes straight through, while the other half is reflected -- this
is useful for interferometry.

Decoration

A
decorative reflecting sphere of thin metal-coated glass,
working as a reducing wide-angle mirror, is sold as a Christmas tree decoration
called a bauble.

Signaling

With
the sun as light source, a mirror can be used to signal, by variations in the
orientation of the mirror. The signal can be used over long distances, possibly
up to 60 kilometers on a clear day. This technique was used by Native American tribes
and numerous militaries to transmit information
between distant outposts.